Is brick a good thermal insulator

Big Bang HTL 1, textbook

9we2z7 128 Selected phenomena In addition to free convection, there is also forced convection. This means that the convection movement is generated by ventilation or pumps. This is the case with water in central heating or in the radiator of a car. Forced convection is also the most important mechanism for transporting heat from inside the body to the surface of the skin (see Section 15.4). Preventing convection If convection is prevented, air is an extremely good heat insulator (see Tab. 15.1, Chapter 15.1). The fact that our clothing warms is less due to the material itself than to the air that “holds” the material. Therefore, the thermal conductivity of wool, feathers and fur is practically just as small as that of air. Clothes prevent convection (F7). A fur would be particularly warm if you carried it inside, because then the air is held even better! Polar bears cannot wear their fur inside, but their hair is filled with air, making them perfect to prevent convection. The principle of preventing convection is also used in building (F11). Insulation materials always work with the help of trapped air, which is prevented from circulating (Fig. 15.7 c). With a double window (a, F10), a lot of heat is lost through convection. In a double-glazed window (b), rising and falling air obstruct each other and convection is practically prevented. i Fig. 15.7: a) double window, b) double glass window and c) hollow brick with enclosed air Fig. 15.8: The Gulf Stream is a huge convection cylinder. It carries 30 times more water per second than all the rivers in the world combined. In around 2 hours it transports as much energy (2 · 10 18 J) as is needed in all of Austria in one year! Summary Convection is the circulation of liquids or gases. The trigger is natural (gravity) or artificial (pump or ventilation). 15.3 Visible temperatures Thermal radiation The third form of heat transfer is thermal radiation. Heat conduction and convection are bound to matter, there is also heat radiation in a vacuum. Z Stars have different colors when viewed through the telescope! But what is the difference between a red and a blue star? An ice cube and a thermometer are located at the focal points of two parabolic mirrors (Fig. 15.10). What is happening and why And what if there was a glowing coal on the left? L How cold is it in space? Can one even speak of a certain temperature there? With a magnifying glass you can focus the light of the sun so that you can light matches and paper. How hot can the point get? Hotter than the sun? Can you justify that L F13 A2 Fig. 15.9 Fig. 15.10 F14 A2 F15 A2 F16 A2 The thermal movement of the particles of an object (see Tab. 14.1, Chapter 14) produces thermal radiation or thermal radiation. Only an object that would have exactly 0 Kelvin would not radiate. But there are no such objects. Therefore one can say: Every object in the universe emits heat rays. Heat rays belong to the electromagnetic waves (see Fig. 12.7 and point 2 in Section 12.2). They cover a broad spectrum of wavelengths and have a temperature-dependent radiation maximum. The hotter the object, the shorter the radiation maximum (see Fig. 15.11). For testing purposes only - property of the publisher öbv

Made with FlippingBook

RkJQdWJsaXNoZXIy ODE3MDE =